Epidemiological studies have suggested that tea may have a protective role in disease, including certain human cancers. Catechin polyphenols isolated from green tea have been shown to inhibit proliferation of cultured mammalian cells including colon carcinoma, lung carcinoma, breast carcinoma, melanoma, and leukemic cells (Lea, M. A. et al. 1993. Cancer Lett. 68:231-236; Valcic, S. et al. 1996. Anticancer Drugs 7:461-468). It has been reported that a major green tea catechin polyphenol, (−)epigallocatechin gallate (EGCG), inhibits growth of human tumor cells, including Caco-2 colorectal cancer cells, Hs578T breast cancer cells, and SV40-transformed W138 cells, but has little or no inhibitory effect on the growth of their normal counterparts (Chen, Z. P. et al. 1998. Cancer Lett. 129:173-179).
Black tea extracts have also been shown to be potent in inhibiting tumorigenesis in several animal model systems, including skin (Javed, S. et al. 1998. Biomed. Environ. Sci. 11:30-7-313), lung (Yang, G. Y. et al. 1997. Carcinogenesis 18:2361-2365), colon (Weisburger, J. H. et al. 1998. Carcinogenesis 19:229-232), esophagus (Morse, M. A. et al. 1997. Nutr. Cancer 29:7-12), and mammary gland (Rogers, A. E. et al. 1998. Carcinogenesis 19:1269-1273). The major black tea polyphenols have been characterized to be theaflavin (TF-1), theaflavin-3-gallate and theaflavin-3′-gallate mixture (TF-2), and theaflavin-3,3′-digallate (TF-3). These theaflavin polyphenols are fermentation products derived from green tea polyphenols and are responsible for the characteristic color, fragrance and taste of black tea.
The biological effects of each individual black tea polyphenol have not been well-studied in terms of their molecular mechanisms. TF-3 has been shown to be as potent as EGCG from green tea inhibiting the growth of human A431 carcinoma cells and in reducing autophosphorylation of EGF and PDGF receptors (Liang, Y. C. et al. 1999. Carcinogenesis 20:733-736). There are no reports of the activity of any other black tea polyphenols.